1. Field of the Invention
The present invention relates to a power transfer device in combination with a power transmission for automotive vehicles, and more particularly to a power transfer device of the type which includes a limited-slip differential arranged to effect limited-slip torque proportional to the difference in rotational speed between the front and rear wheel axles of the vehicle.
2. Discussion of the Background
In Japanese Patent Early Publication No. 60-236839 issued on Nov. 25, 1985, there has been proposed a power transfer device which includes a center differential of the bevel gear type arranged to split the driving torque from a power transmission into two torque delivery paths respectively for front-wheel drive and rear-wheel drive, a front-drive differential of the bevel gear type arranged coaxially with the center differential at one side thereof to transfer the split driving torque from a first side gear of the center differential to front-wheel axles, and an output ring gear for rear-wheel drive arranged coaxially with the center differential at the other side thereof to transfer the split driving torque from a second side gear of the center differential to rear-wheel axles. In the power transfer device, a limited-slip differential in the form of a viscous coupling is assembled within a mounting case for the output ring gear to effect limited-slip torque proportional to the difference in rotational speed between the side gears. The limited-slip torque causes an increase of the driving torque at a low speed side and a decrease of the driving torque at a high speed side.
The limited-slip differential is, however, conditioned to be always driven in operation of the power transfer device. This means that when the power transfer device is operated in a condition where the vehicle is placed on a chassis dynamometer at its front or rear road wheels for measurement of emission gas, rate of fuel consumption, driving torque or the like and for check of meter indication, the limited-slip differential is driven for a long period of time under a large difference in rotational speed between the side gears of the center differential caused by rotation of the front or rear road wheels. This causes frictional heat in the limited-slip differential and results in rise of fluid temperature in the limited-slip differential. When the vehicle is towed in a condition where it is suspended from a tractor at its front or rear road wheels, the power transfer device is operated by rotation of the rear or front road wheels during travel of the tractor. In such a situation, the limited-slip differential is driven for a long period of time under a large difference in rotational speed between the side gears of the center differential. This causes the same problem described above. Furthermore, when the vehicle is towed in a condition where the front or rear wheel axles are suspended from a tractor, the power transfer device acts to rotate the front or rear road wheels during travel of the tractor.